Page last updated at 23:57 GMT, Monday, 3 November 2008

Antelope's sex signal in the knee

Eland bull (Jakob Bro-Jorgensen)
Male eland antelope click a tendon in their knees to establish dominance

To demonstrate their sexual prowess, peacocks spread their tail feathers, men flex their muscles and eland antelope, it seems, click their knees.

Scientists from the Zoological Society of London and the University of Copenhagen recorded the sounds of eland bulls in Kenya, Africa.

Reporting in the journal BMC Biology, the researchers say that the depth of the sound correlates to body size.

The tactic signals the bulls' fighting potential, establishing mating rights.

The sound is thought to be made as a tendon in the animals' legs slips over one of the leg bones, and can be heard from hundreds of metres away.

"The tendon in this case behaves like a string being plucked, and the frequency of the sound from a string correlates negatively with both its length and diameter," said Jakob Bro-Jorgensen.

That means that the sound signals how large - and thus how fighting fit - the antelopes are. The antelope can thus establish mating rights among each other while avoiding actual fights.

The unusual approach adds to the list of signals that are known in the animals to provide an indication of their status.

A fold of skin under their throats called the dewlap indicates age, and the darkness of their hair indicates levels of aggression.

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